We presently have an ultrahigh-energy defibrillator for delivering unidirectional rectangular, trapezoidal, truncated exponential, or untruncated exponential waveforms. It utilizes two 18,000 joule capacitor banks. One of the banks can be patched to deliver up to 800, 1600, or 2400 volts; the other bank can be patched for up to 5,000 10,000 or 15,000 volts. The banks are electronically switched with SCR's or hydrogen thyratrons. We intent to build an add-on unit which, when combined with our present defibrillator, will emable us to deliver high energy bidirectional retangular, trapezoidal, truncated exponential, or untruncated exponential waveforms. The add-on unit will use another 18,000 joule capacitor bank which can be patched for 2,500, 5,000, or 7,500 volt operation. Strings of SCR's will be used for switching the capacitor bank in the new unit. Our new bidirectional defibrillation capability will be used to generate extensive data in 100 kg calves from which a family of curves of per cent success versus pulse duration for bidirectonal (one cycle) rectangular waveforms having current amplitudes of plus or minus 35, plus or minus 50, plus or minus 70, plus or minus 100, and plus or minus 140 amperes can be plotted. This data, when combined with our earlier results with corresponding unidirectional waveforms, should enable us to critically compare the effectiveness of unidirectional and bidirectional waveforms in acheiving transthoracic ventricular defibrillation.